Forensic Evidence and Crime Scene Investigation

📖 Pattu Rajan v. State of Tamil Nadu, (2019) 4 SCC 771

Facts: A conviction rested in part on the opinion of experts, and the question arose as to the weight such opinion carries, the extent to which a court is bound by it, and what is required before it can be acted upon.

Held: The Supreme Court held that the opinion of an expert is a piece of evidence and no more: it is admitted because the subject lies beyond the ordinary knowledge of the Court, but it is not binding, and the Court remains the judge of the fact to which the expert speaks. It held that the value of expert testimony depends on the reasons stated in support of the conclusion, and that an opinion offered without disclosing its basis cannot be tested and carries little weight. The Court held that an expert must be qualified in the field, that the field must be one which admits of expert opinion at all, and that the Court should examine how the data were obtained, what method was applied, and whether the conclusion fits the rest of the evidence or stands alone against it. It emphasised that corroboration is not a rule of law but a rule of prudence, and that a court acting on expert opinion alone should be satisfied that it is cogent and reliable.

Ratio: Expert opinion is evidence and no more — admitted because the subject is beyond ordinary knowledge, but not binding. Its value depends on the reasons disclosed in support, and an opinion offered without its basis cannot be tested.

📖 Sharad Birdhichand Sarda v. State of Maharashtra, (1984) 4 SCC 116

Facts: A conviction rested entirely on circumstantial evidence, and the Supreme Court took the occasion to state, in a form which has been applied ever since, the conditions which must be satisfied before a conviction may be founded on such evidence.

Held: The Supreme Court laid down five conditions, described as the panchsheel of a case of circumstantial evidence. The circumstances from which the conclusion of guilt is drawn should be fully established, and the facts so established should be consistent only with the hypothesis of the guilt of the accused. The circumstances should be of a conclusive nature and tendency; they should exclude every possible hypothesis except the one to be proved; and there must be a chain of evidence so complete as not to leave any reasonable ground for a conclusion consistent with the innocence of the accused, and must show that in all human probability the act must have been done by him. The Court emphasised the distinction between may be proved and must be or should be proved, holding that a grave suspicion is not a substitute for proof, however strong it may be, and that a court must not allow the gap between the two to be bridged by conjecture.

Ratio: A conviction on circumstantial evidence requires the circumstances to be fully established, consistent only with guilt, conclusive in nature, exclusive of every other hypothesis, and forming a chain so complete as to leave no reasonable ground for innocence.

⚠ Where forensic evidence fails, and why

It was never collected, the scene having been disturbed before anyone competent attended. Section 176(3) attacks this directly.

The chain of custody is broken — nobody can say where the sample was between seizure and examination. Sections 105 and 185 attack this, by recording the seizure and placing the record in judicial custody at once.

The connection with the scene cannot be proved, so that a faultless laboratory report establishes nothing about the case. The videography under section 176(3) is directed at exactly this.

Delay has degraded the material; the report is tendered without the expert, so its basis cannot be tested; or the court treats it as conclusive rather than as evidence to be weighed.

And where scientific evidence could have been obtained and was not, the omission tells against the prosecution — the court being entitled to ask why the obvious step was not taken.

The position stated shortly

1. Forensic evidence fails more often at the scene than at the laboratory. The science is rarely in doubt; the provenance of the material usually is.

2. Section 176(3): for an offence punishable with seven years or more, a forensic expert shall visit the scene and the collection shall be videographed.

3. Sections 105 and 185: the search and seizure process, including the list and its signing, shall be recorded and forwarded to a Magistrate without delay.

4. The chain of custody must be accounted for by evidence. A perfect report proves nothing if the Court cannot be told where the sample was.

5. The report is the opinion of a witness, not a verdict. It must be proved, its basis explained, and the expert cross-examined where disputed.

6. Expert opinion is not binding, the Court remaining the judge of the fact; its value depends on the reasons disclosed: Pattu Rajan.

7. An opinion offered without its basis cannot be tested and carries little weight.

8. Where the case rests on circumstantial evidence, the circumstances must be fully established, consistent only with guilt, conclusive, and form a complete chain: Sharad Birdhichand Sarda.

9. Grave suspicion is not a substitute for proof, and the gap between may be proved and must be proved is not bridged by conjecture.

10. Where scientific evidence could have been obtained and was not, the omission tells against the prosecution, and the Court may ask why the obvious step was not taken.